This invention relates to a vehicle skid control system, and more particularly to a "pumping-type" or "pulse action" vehicle brake control system that advantageously utilizes in combination: (1) Speed Sensor means; (2) Frequency Convertor means; (3) Slip and Ramp Signal Generator and Comparator Circuit means; (4) D.C. Level Detector and Zero Velocity Gate circuit means; (5) Acceleration and Deceleration Differentiator circuit means; (6) Variable Threshold Acceleration and Deceleration Gate Circuit means; (7) Acceleration Programmed Oscillator circuit means; (8) Fixed Threshold Variable Duty Cycle Pulse Generator circuit means; (9) Reset Timer and Gate Circuit means; (10) Brake Control Logic circuit means; and (11) Failsafe Monitoring and Indicating means.
When the operator of a land vehicle desires to stop his vehicle under emergency braking conditions or under adverse road conditions, a realistic probability exists that the land vehicle will undergo an uncontrolled skid, or a controllable skid which prevents the operator from bringing his land vehicle to a safe stop within the distance available. Under these circumstances, one factor that indicates an imminent skid is the relative relationship between vehicle wheel speed and vehicle speed. It has been readily accepted by safety experts and prefessional land vehicle drivers that vehicle stability can be achieved by automatically "pumping" or "pulsing" of the brakes associated with the wheels of the land vehicle in a pre-programmed manner dynamically related to the rate of acceleration and deceleration of the land vehicle.
In more recent times, several systems have been developed that operate on the principle of selectively inhibiting the normal braking action initiated by the operator of the land vehicle. In one of these systems, wheel speed sensors are utilized to generate A.C. signals that are proportional to vehicle wheel speed. The vehicle wheel speed signals are then processed through a control module which generates a D.C. voltage to energize a solenoid in an actuator that controls the hydraulic braking system to the vehicle wheels. When a skid is imminent, a command signal from the control module causes the actuator solenoid to close a vacuum passage in a diaphragm chamber. By action of the diaphragm and the normal hydraulic pressure from the master cylinder of the land vehicle braking system as developed by the operator thereof, the hydraulic pressure to the vehicle wheels is released, thereby inhibiting the vehicle wheel braking action. When the vehicle wheels spin up to the vehicle speed or to a selective lower speed, the control module produces a signal to deenergize the actuator solenoid. This in turn restores line pressure in the vehicle braking system and reapplies the vehicle brakes. In effect, what is achieved by this system is that the vehicle brakes are "pumped" or "pulsed" in a manner often recommended for controlled braking under adverse driving conditions.
The control module of the aforementioned system includes a frequency convertor for each wheel speed sensor for converting a frequency varying signal into a varying direct current signal proportional thereto. A summation of the outputs from each of the frequency convertors produces a composite of the vehicle wheel speed signals. Respective deceleration and acceleration rate detectors respond to the output of the summing amplifier for producing outputs proportional to the rate of deceleration and acceleration respectively of the wheels of the land vehicle. A signal proportional to the output of the summing amplifier is transferred to a vehicle velocity ramp generator and one input of an automatically adjustable switching circuit. The vehicle velocity ramp generator produces a step function ramp signal having an overall slope related to the actual speed of the land vehicle when braking to a stop. The automatically adjustable switching circuit also receives an input from the vehicle velocity ramp generator and an input from a retarding force detector, and produces an output whenever the summation of the wheel speed signal, the velocity ramp signal, and the retarding force signal reaches a first threshold condition. The retarding force detector produces an output signal which relates to the braking factors including tire condition, brake condition, and the condition of the road surface. To generate a control signal to the actuator solenoid, the output of the deceleration rate detector, the acceleration rate detector, and the automatically adjustable switching circuit must have a certain designated relationship. These three signals are the inputs to the brake controller as the last component in the control module.
A more detailed description of the features briefly stated above with regard to a prior known vehicle skid control system is set forth in a copending patent application, Ser. No. 25,131, filed Apr. 2, 1970, for ""VEHICLE SKID CONTROL SYSTEM," which is assigned to the assignee of this application.
Another known and similar system that operates on the principle of inhibiting the normal braking action initiated by the operator of the land vehicle is set forth in U.S. Pat. No. 3,578,819, issued May 18, 1971, for "SKID CONTROL SYSTEM," in the name of Thomas M. Atkins.